feat: Phase 1 Group 1 — add account_id to core session tables

Migration sequence: add nullable → backfill via user_id/ai_session chain
→ verify zero NULLs → SET NOT NULL → CREATE INDEX.

Tables: sessions, attachments, session_supporting_data,
        session_resolution_outputs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/session.py

@@ -31,6 +31,12 @@ class Session(Base):
         nullable=False,
         index=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     tree_snapshot: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
     path_taken: Mapped[list[str]] = mapped_column(JSONB, nullable=False, default=list)
     decisions: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)